Structural fatigue can be defined as the failure of a material due to the progressive growth of minute cracks under cyclic loading. The fatigue life of a plant structure is, in general words, the time to failure under a particular cyclic loading environment. The evaluation of the fatigue life consumption is an important part of plants design and calculation, but should be also controlled during the plant's operational life, by means of a so-called Structural Fatigue Monitoring System.
A plethora of monitoring systems has been conceived to evaluate the fatigue life consumption of aircraft structures. They have been used traditionally in military aviation and more recently in certain applications of civil aviation. There are two main advantages of this kind of systems: to ensure the safe operation of the aircraft and to reduce the costs of ownership by optimizing aircraft usage and maintenance tasks during the whole operational life.
Most of the fatigue monitoring systems include some characteristics that allow classifying them into groups according to following three main features: system philosophy, technique basis and concept of application.
The philosophy defines the scope of the system. The fatigue monitoring systems can be divided into two groups according to their philosophy, damage detection and damage prognosis. The aim of the systems under the damage detection group is to locate and measure the position and severity of the eventual damages (coming from structural fatigue or from any other source like corrosion, accidental, etc.). On the other side, the systems belonging to the damage prognosis category estimate the position and/or severity of the possible damages selected from a predefined set and considering a particular aircraft usage.
The technique basis determines what kind of variables are going to be used by the system in order to either detect or forecast the damage. Two main groups can be identified: direct techniques and parametric techniques. The direct systems measure directly in the structure some physical variables that can be used without the aid of an external model. This is an inductive technique because the system makes global assumptions from a set of particular data. For example, the system can include a number of strain sensors to measure the strains at some locations of the structure, and use that information to perform fatigue and damage tolerance calculations. The parametric systems use global operational parameters of the aircraft to feed a particular model and obtain the necessary data. This is a deductive technique because it makes particular assumptions about the structure based on general measurements. For example, flight cycles and flight hours may be used to control aircraft usage and apply the maintenance program according to a set of aircraft sortie profile codes.
Both techniques have advantages and disadvantages. Direct systems are accurate and precise because the variables used for the location (damage detection systems) or crack initiation and crack growth calculations (damage prognosis systems) are directly measured from the structure, but the installation and maintenance cost of the sensors is usually high. On the other hand, parametric systems are, in general, less accurate and precise due to the need of using an external model to obtain useful data (actually, nowadays the main challenge to obtain a reliable parametric system is the complexity of developing an accurate model to process the information), but they are less expensive than the former as in many cases the data come from other systems already installed in the aircraft.
Historically, parametric systems were developed earlier than direct systems due to their simplicity (e.g., vertical load factor exceedances counters). Once the technology evolved, increasingly sophisticated recorders began to be installed on-board (e.g., strain data recorders), and direct systems began to be used, being the preferred concept for years. During the last two decades parametric systems have been used again due to the improvement in the models that process parametric data and in the computation capabilities.
Finally, there are three application concepts depending on the number of aircraft that are monitored and the period of time during which they are tracked:                Individual Aircraft Tracking (IAT), where every aircraft of the fleet is monitored during its whole operational life;        Temporary Aircraft Tracking (TAT) when a limited number of aircraft are monitored during a limited period of time; and        Selected Aircraft Tracking (SAT) when some aircraft of the fleet are monitored during its whole operational life.        
The current invention presents a prognosis parametric method and system that allows individual aircraft tracking, joining the precision and accuracy of prognosis direct systems and the low cost of previous prognosis parametric systems. The present invention is applicable not only to aircraft, but to any plant structure, such as wind turbines, ships, buildings, bridges or towers, in which global parametric data can be associated to a level of strain in one or several locations of the structure.